Our devs are so cautious, I think they designed Sprite Skills so that the they are not overpowered or would change the game significantly.. with this I think even if we were able to use skills simultaneously/combo them it would still not make much changes in the gameplay. For example, one good combo would be to activate Hexing Haze then Cloak.. however my Lvl 100 Maskeraith's Haunted Haze barely gives significant damage in Tier 3, so its still not overpowered.
Skills are what makes the game enjoyable but instead of enjoying it I feel frustated/annoyed due to the combined cooldown. Please consider devs.
Please also consider this:
Suggested Battle Sprite Update
http://forums.spiralknights.com/en/node/86474
It would be too overpowered. However you should be able to use abilities 3 seconds after an ability ends.